CCD-world: Re: cleaning

[Geoff Evans]

The following was posted to CCD-world:


Here follows e-mails received from Site in 1997 regarding ccd
contamination and cleaning.

----------------------Reply 1------------------------------------------

Regarding cooling a CCD and it's change in color.  We have
recently experienced some difficulty in that department.  We are
building some new test cameras and have been cooling parts in them.
Much to our surprise we found that they turned a reddish color some
where around 245 K and colder and at low pressure.  What we found was
some of the wiring in the dewar used plastic insulated wire.
Replacing this with teflon insulated wiring and thoroughly degreasing
and cleaning the dewar eliminated the problem.  The principle culprit
seemed to be the plasticizers in the insulation condensing on the
chip.  Warming the chip up did not get rid of the residue.  It needed
to be cleaned and baked.

----------------------------Reply 2-------------------------------------

Use the shorted ZIF socket and drip the solvents onto the device.  
However, first I would caution you that acetone has a quite low flash
point.  I think the best strategy is to use the acetone at room temp (20C)
and not heat the part or heat the acetone.  It will do its job at this
temperature.  Second, acetone is a polar solvent so will attach to the Si
surface and leave a residue.  To remove the acetone I would apply it to
the device, filling or partially filling the cavity, and then start
diluting it with the Isopropyl Alcohol, again at 20 C.  Let it spill over
the edges of the package and evaporate.  Once you have rinsed sufficiently
to get rid of the acetone you could then start applying the heated IPA.  
As the final step you could pour out the heated IPA and hold it over the
heated vapors removing it slowly from the stream.

In pouring out the IPA, the one thing that you want to be careful of 
is that capillary action does not cause the bond wires to be pulled 
down to the package.  Sometimes this happens and will clearly cause 
an electrical short.  The wire can be carefully and gently moved away 
from the package, but the device needs to be shorted and you need to 
be grounded.  Working under a microscope is essential.  Repairing the 
bond wire is something we can do if needs be.  It may not be a 
problem.


Geoff Evans  ( gpe at saao.ac.za )
Head Electronics
South African Astronomical Observatory
P.O. Box 9 , Observatory , 7935 , South Africa
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